Earth fault circuit breaker

Abstract: An earth fault circuit breaker (10) for use in a circuit including an AC power supply and a load (11), which circuit breaker comprises a current-to-voltage converter (12) having two transformers for monitoring the current before and after flowing through the load, a leakage current/ voltage detector (13A) for processing the outputs (E1 & E2) of the transformers to produce a resultant voltage, a voltage comparator (16) for comparing the said resultant voltage with a predetermined reference voltage to determine if there is a difference between the two transformer outputs, which is caused by an earth fault appearing at the load, and a switching driver (14) for disconnecting the load from the power supply in response to such a difference between the two transformer outputs exceeding the predetermined reference voltage, wherein each transformer has first and second primary windings (P1 & P2) arranged to be connected to opposite sides of the load and the two transformers are arranged to provide outputs at their respective secondary windings (S), corresponding to the normal load current (I2), in a substantially anti-phase relationship. The circuit breaker (10) may optionally include another voltage comparator (21) to detect overload current by monitoring the output of either one of the transformers.

Description: The present invention relates to an earth fault circuit breaker capable of disconnecting a load from an AC power supply when there is an accidental earth leakage of the load current, and optionally when there is an over-load current. Earth fault circuit breakers for use in an AC power/load circuit are generally known, for example as disclosed in German Patent Application No. P 26 11 674. Such a circuit breaker is formed by a current-to-voltage converter having a transformer for monitoring the current before and after flowing through the load, a leakage current/voltage detector for processing an induced voltage of the transformer to produce a resultant voltage, a voltage comparator for comparing the resultant voltage with a predetermined reference voltage and for generating a signal indicative of a difference therebetween caused by an earth fault appearing at the load, and a switching circuit for disconnecting the load from the power supply in response to such a signal. The invention seeks to provide an improved earth fault circuit breaker. According to the invention, there is provided an earth fault circuit breaker for use in a circuit including an AC power supply and a load, said circuit breaker comprising a current-to-voltage converter having a transformer for monitoring the current before and after flowing through the load, a leakage current/voltage detector for processing an induced voltage of the transformer to produce a resultant voltage, a voltage comparator for comparing the resultant voltage with a predetermined reference voltage and for generating a signal indicative of a difference therebetween caused by an earth fault appearing at the load, and a switching circuit for disconnecting the load from the power supply in response to such a signal, characterised in that: the current-to-voltage converter has first and second transformers for monitoring the current flowing into the load and current flowing from the load, each transformer having first and second primary windings connected to opposite sides of the load and including a secondary winding in which a voltage is to be induced, the first and second transformers being arranged such that the induced voltages of the first and second transformers are in a substantially anti-phase relationship; the leakage current/voltage detector is arranged to process the induced voltages of the first and second transformers to produce a resultant voltage; and the voltage comparator is arranged to compare the resultant voltage with a predetermined reference voltage and to generate a signal indicative of a difference between the induced voltages of the first and second transformers for the switching circuit to operate. In a preferred arrangement, the corresponding first primary windings of the first and second transformers are connected together inseries and the corresponding second primary windings of the first and second transformers are connected together in series. In another preferred arrangement, the corresponding first primary windings of the first and second transformers are connected together in parallel and the corresponding second primary windings of the first and second transformers are connected together in parallel. In a first preferred embodiment, the first and second primary windings of each transformer have substantially the same wire cross-sectional area but different numbers of turns. In a second preferred embodiment, the first and second primary windings of each transformer have substantially the same number of turns but different wire cross-sectional areas. In a third preferred embodiment, the first and second primary windings of each transformer have substantially the same wire cross-sectional area and number of turns but one of the first and second primary windings is connected in parallel with a resistor. Advantageously, the earth fault circuit breaker further comprises means for adjusting the predetermined reference voltage to change the sensitivity of the circuit breaker responding to an earth fault. The earth fault circuit breaker may further comprise a second voltage comparator for comparing the voltage across the secondary winding of one of the transformers with a predetermined threshold voltage, whereby the switching circuit is operated in order to avoid an over-current flowing through the load when the voltage across said secondary winding exceeds the predetermined threshold voltage. Advantageously, the earth fault circuit breaker further comprises means for changing the predetermined threshold voltage of the second voltage comparator to adjust the sensitivity of the circuit breaker. The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of an embodiment of an earth fault circuit breaker according to the invention; Figures 2A to 2E are schematic circuit diagrams of five possible current-to-voltage converters suitable for use in the circuit breaker of Figure 1; Figures 3A to 3C are waveform diagrams showing the load and leakage currents detected by each current-to-voltage converter of Figures 2A to 2E; and Figures 4A and 4B are schematic circuit diagrams of two possible leakage current/voltage detectors suitable for use in the circuit breaker of Figure 1. Referring firstly to Figure 1 of the drawings, there is shown an earth fault circuit breaker 10 embodying the invention, which is used in a circuit including a load 11 and an AC power supply (not shown). The circuit breaker 10 comprises a current-to-voltage converter 12, to which the load 11 is connected, for monitoring the load current including any earth fault current leaking from the load 11. A leakage current/voltage detector 13A is connected to the output of the converter 12 for processing the converter output voltage to provide a resultant voltage for checking against a predetermined reference voltage to see if there is an earth leakage current. The circuit breaker 10 further includes a switching driver 14 arranged to disconnect the load 11 from the power supply in response to the presence of an earth leakage current. Figures 2A to 2E show five alternative circuit designs to implement the current-to-voltage converter 12, of which the converter 12A of Figure 2A is firstly referred to. The converter 12A is formed by first and second transformers T1 and T2. The transformer T1 has first and second primary windings P1 and P2, and the transformer T2 has first and second primary windings P2 and P1. The corresponding primary windings P1 of the transformers T1 and T2 are identical, and so are the respective primary windings P2. The transformers T1 and T2 have corresponding identical secondary windings S. The first primary windings P1 and P2 of the transformers T1 and T2 are connected in series on the LIVE side of the load 11, and the second primary windings P2 and P1 on the load NEUTRAL side. It is to be understood that the transformers T1 and T2 have the same construction, but the connection of the primary windings P1 and P2 of one transformer T1 with respect to the load 11 is opposite to that for the other transformer T2. In Figure 2A, the normal load current is designated by I2 and the leakage current caused by an earth fault appearing at the load is designated by I1. The load current I2 flows through both primary windings P1 and P2 of each transformer T1 or T2, but the leakage current I1 will only flow through, as shown, the first primary winding P1 of the transformer T1 and the first primary winding P2 of the transformer T2. The primary windings P1 and P2 of each transformer T1 or T2 have the same wire cross-sectional area, but the primary winding P1 has a relatively larger number of turns than the primary winding P2. The relative number of turns of the magnetically-coupled primary windings P1 and P2 is a factor determining the North/South polarity of the transformer T1 or T2. The configuration and arrangement of the primary windings P1 and P2 of the transformers T1 and T2 are designed such that the respective transformer inductances have the same magnitude but substantially in an anti-phase relationship. The load current I2 passing through the transformers T1 and T2 will cause substantially opposite North/South polarization effect in the primary windings P1 and P2. The North/South polarity of the magnetic field of each transformer T1 or T2 at any given time is determined by the resultant magnetic induction of one primary winding P1 or P2 over or below the other primary winding P2 or P1. Insofar as the load current I2 is concerned, this results in output emf's E1 and E2 at the corresponding secondary windings S of the transformers T1 and T2 having the same magnitude but 180° out of phase, as shown in Figure 3B. As the leakage current I1 flows only through the first primary windings P1 and P2 but not the second primary windings P2 and P1 of the transformers T1 and T2, respectively, the emf's E1 and E2 induced by the leakage current I1 at the transformer secondary windings S remain in phase, as shown in Figure 3A. The combined waveform of the emf E1 or E2 induced by the load and leakage currents I2 and I1 for each transformer T1 or T2 is shown in Figure 3C. Figures 4A and 4B of the drawings show two alternative designs 13A and 13A' for the leakage current/voltage detector, the output of which is connected to a voltage comparator block 13B. The leakage current/voltage detector 13A is formed by a series pair of filtering capacitors C1 and C2 connected across the seconding windings S of the transformers T1 and T2, respectively, and by a resistor R3 connected in parallel with the pair of capacitors C1 and C2. As the capacitors C1 and C2 are connected in series, the filtered/ rectified output emf's E1 and E2 of the transformers T1 and T2 are superimposed together to produce a resultant voltage appearing across the resistor R3 for the voltage comparator block 13B to process. It is to be understood that the two emf's induced by the load current I2 alone are substantially cancelled out by reason of their anti-phase relationship, resulting in the said resultant voltage representing a doubled value of the leakage current I1. The voltage comparator block 13B comprises an op-amp (operational amplifier) amplifier 15 for amplifying the resultant voltage given by the leakage current/voltage detector 13A and an op-amp comparator 16 for comparing the amplified resultant voltage with the reference voltage which is predetermined but adjustable by means of a variable resistor R4. The reference voltage corresponds to the maximum tolerable limit of an earth fault leakage current. Thus the triggering sensitivity of the circuit breaker 10 responding to an earth fault is adjustable. The op-amp comparator 16 has at its output a diode D1 which conducts when the amplified resultant voltage exceeds the reference voltage. The switching driver 14 is formed by a solenoid 17 controlling a spring-biassed hinged soft iron arm 18 and a thyristor 19 controlling the operation of the solenoid 17. The diode D1 of the op-amp comparator 16 is connected to the gate terminal of the thyristor 19 such that when the diode D1 conducts (in response to occurrence of leakage current I1) to trigger the thyristor 19 via its gate terminal, the thyristor 19 conducts to permit energizing of the solenoid 17. The solenoid 17 then attracts to pivot the hinged arm 18 forwards to open a double-pole/double-throw switch 20 controlling the supply of power from the power supply to the load 11. It is to be appreciated that the leakage current I1 may occur at any part of the load circuit, resulting in an unbalance or unsymmetrical state between the outputs of the transformers T1 and T2 and thereby producing a resultant voltage to trip the switch 20 by means of the switching driver 14. The circuit breaker 10 further includes another voltage comparator block 21, incorporating an op-amp comparator 22, for detecting any over-current flowing through the load 11. A first input terminal of the op-amp comparator 22 is connected to the secondary winding S of the transformer T2 (or to that of the transformer T1) for picking up the output voltage of the transformer T2 (or transformer T1), said output voltage representing the magnitude of the load current I2. A second input terminal of the op-amp comparator 22 is connected to a variable resistor R5 provided across a DC operating voltage V for the entire circuitry and the local earth. The variable resistor R5 determines an adjustable threshold voltage for comparison by the op-amp comparator 22 with the voltage picked-up by the first input terminal. When the picked-up voltage exceeds the threshold voltage, the op-amp comparator 22 provides an output through a diode D2 connected to the gate terminal of the thyristor 19, thereby triggering the thyristor 19 to operate the solenoid 17 to open the switch 20 in order to disconnect the load 11 from the power supply. It is appreciated that by means of the variable resistor R5, the sensitivity of the circuit breaker 10 responding to a load over-current is adjustable. The switch 20 has a pair of spring-biassed moving switch contacts 25 and a elongate operating member 26 bearing at one end on the switch contacts 25. The opposite end of the operating member 26 has a hook formation 27 normally engaging behind a fixed abutment 28 to hold the switch 20 closed. The hinged arm 18 is pivoted by the solenoid 17 to push the hook formation 27 off the abutment 28 to open the switch 20. After the earth fault or over-current has been rectified, the switch 20 is reset to re-connect the load 11 to the power supply by manually pushing the operating member 26 longitudinally back until the hook formation 27 engages behind the abutment 28 again, thereby re-closing the switch 20. The circuit breaker 10 further includes a built-in voltage regulator 23 for converting the mains AC voltage from the power supply into a DC voltage V for operating the entire circuitry. The regulator 23 is formed by a bridge rectifier 24, two capacitors C3 and C4, a resistor R6 and a zener diode Z1 connected together in a conventional manner to perform full wave rectification. Referring more specifically to Figure 2B showing a second current-to-voltage converter 12B which operates generally in the same manner as the first converter 12A. In this converter 12B, the primary windings P1 and P2 of both transformers T1 and T2 have the same number of turns but the primary windings P2 have a relatively larger wire cross-sectional area than the primary windings P1. Insofar as each pair of magnetically-coupled primary windings P1 and P2 is concerned, these windings interact to determine a certain phase shift for the transformer output emf E1 or E2 induced by the load current I2 flowing through both windings. The corresponding pairs of primary windings P1 and P2 of the transformers T1 and T2 are arranged such that the respective load current-induced transformer output emf's E1 and E2 are directly 180° out of phase. Insofar as the leakage current I1 is concerned, the corresponding induced transformer output emf's E1 and E2 remain in phase as the leakage current I1 only flows through the first primary windings but not the second primary windings of the transformers T1 and T2. More specific reference is now made to Figure 2C, showing a third current-to-voltage converter 12C which again operates generally in the same manner as the first converter 12A. All the four primary windings P1 and P2 of the transformers T1 and T2 have the same wire cross-sectional area and the same number of turns, but a resistor R1 or R2 is connected in parallel with each primary winding P2 to determine the required phase shift for the transformer output emf's E1 and E2. Figures 2D and 2E show two other alternative current-to-voltage converters 12D and 12E which are very similar to the first and second converters 12A and 12B, respectively, except that the two first or second primary windings P1 and P2 are connected together in parallel rather than in series.

Claims: An earth fault circuit breaker (10) for use in a circuit including an AC power supply and a load (11), said circuit breaker (10) comprising a current-to-voltage converter (12) having a transformer (T1/T2) for monitoring the current before and after flowing through the load (11), a leakage current/voltage detector (13A) for processing an induced voltage of the transformer (T1/T2) to produce a resultant voltage, a voltage comparator (13B) for comparing the resultant voltage with a predetermined reference voltage and for generating a signal indicative of a difference therebetween caused by an earth fault appearing at the load (11), and a switching circuit (20) for disconnecting the load (11) from the power supply in response to such a signal, characterised in that: the current-to-voltage converter (12) has first and second transformers (T1 & T2) for monitoring the current flowing into the load (11) and current flowing from the load (11), each transformer (T1/T2) having first and second primary windings (P1 & P2) connected to opposite sides of the load (11) and including a secondary winding (S) in which a voltage is to be induced, the first and second transformers (T1 & T2) being arranged such that the induced voltages of the first and second transformers (T1 & T2) are in a substantially anti-phase relationship; the leakage current/voltage detector (13A) is arranged to process the induced voltages of the first and second transformers (T1 & T2) to produce a resultant voltage; and the voltage comparator (13B) is arranged to compare the resultant voltage with a predetermined reference voltage and to generate a signal indicative of a difference between the induced voltages of the first and second transformers (T1 & T2) for the switching circuit (20) to operate. An earth fault circuit breaker (10) as claimed in claim 1, characterised in that the corresponding first primary windings (P1) of the first and second transformers (T1 & T2) are connected together in series and the corresponding second primary windings (P2) of the first and second transformers (T1 & T2) are connected together in series. An earth fault circuit breaker (10) as claimed in claim 1, characterised in that the corresponding first primary windings (P1) of the first and second transformers (T1 & T2) are connected together in parallel and the corresponding second primary windings (P2) of the first and second transformers (T1 & T2) are connected together in parallel. An earth fault circuit breaker (10) as claimed in claim 1, characterised in that the first and second primary windings (P1 & P2) of each transformer (T1/T2) have substantially the same wire cross-sectional area but different numbers of turns. An earth fault circuit breaker (10) as claimed in claim 1, characterised in that the first and second primary windings (P1 & P2) of each transformer (T1/T2) have substantially the same number of turns but different wire cross-sectional areas. An earth fault circuit breaker (10) as claimed in claim 1, characterised in that the first and second primary windings (P1 & P2) of each transformer (T1/T2) have substantially the same wire cross-sectional area and number of turns but one of the first and second primary windings (P1 & P2) is connected in parallel with a resistor (R1/R2). An earth fault circuit breaker (10) as claimed in claim 1, further comprising means (R4) for adjusting the predetermined reference voltage to change the sensitivity of the circuit breaker (10) responding to an earth fault. An earth fault circuit breaker (10) as claimed in claim 1, further comprising a second voltage comparator (21) for comparing the voltage across the secondary winding (S) of one of the transformers (T1 & T2) with a predetermined threshold voltage, whereby the switching circuit (20) is operated in order to avoid an over-current flowing through the load (11) when the voltage across said secondary winding (S) exceeds the predetermined threshold voltage. An earth fault circuit breaker (10) as claimed in claim 8, further comprising means (R5) for changing the predetermined threshold voltage of the second voltage comparator (21) to adjust the sensitivity of the circuit breaker (10).

Inventors: LAM SHEIR CHUN, LAM, SHEIR CHUN
Applicants: LAM SHEIR CHUN, TAM PUI YIN, LAM, SHEIR CHUN
Application year: 1994
Main classification: H02H3
IPC codes: H02H11, H02H1, H02H3